In recent years, along with a trend toward stricter regulations on exhaust-gas emissions from automobiles or the like, combustion efficiency of an internal combustion engine has been required to be improved.
In general, in any of injection holes of a fuel injection valve, as a fuel liquid film spreads thinly in the injection hole, particle diameter of a droplet formed by breakup of the liquid film after the injection becomes smaller. However, an angle of the liquid film injected from a single injection hole (hereinafter referred to as “single spray angle”) becomes larger. At this time, the spray particle diameter and the single spray angle have a tradeoff relationship as indicated on the vertical axis and the horizontal axis of FIG. 13.
Hitherto, in an internal combustion engine, which injects the fuel toward an intake valve provided to a distal end of an intake port, the fuel is caused to adhere to the intake valve and the vaporized fuel is supplied to a combustion chamber.
However, when a spray angle of a spray aggregate obtained by aggregating a plurality of the single sprays is set too large after the single spray angle is increased with insufficient atomization, the amount of spray, which is located on the outer side with respect to a central axis of the spray, adhering to an inner wall of the intake port increases at the time of injection. Therefore, there is a problem in that engine controllability is lowered.
Moreover, when the spray adhering to the intake port becomes the liquid film, which then flows along the inner wall of the intake port into the combustion chamber, there is a problem in that the degradation of the exhaust-gas emission performance and lowered combustion efficiency are caused.
As solutions to the above-mentioned problems, a variety of technologies to realize both the atomization and the aggregation of the sprays have been proposed (see, for example, Patent Literatures 1 and 2).
In Patent Literature 1, a plurality of injection holes formed through an injection-hole plate are formed on a radially outer side of a valve-seat opening inner wall that is a portion having a minimum inner diameter of the valve seat with a diameter reduced toward a downstream side. A cavity that brings the valve-seat opening inner wall and the injection holes into communication with each other is formed in a downstream-side end surface of the valve seat.
Moreover, when a plurality of injection-hole inlets are projected onto a plane that is perpendicular to a valve-seat axial center, a distance between an outermost diameter of the plurality of injection holes and an outer circumferential wall of the cavity is set equal to or larger than the diameter of the injection hole.
In this manner, when the valve is opened, a fuel flow from a valve-seat axial center toward the center of the injection hole and a fuel flow corresponding to a part of the flow along an outer circumferential surface of the cavity through portions between the injection holes into the injection holes are caused to collide against each other to cause a turbulence in the fuel flow, thereby breaking up the liquid film so as to realize promotion of the atomization.
In Patent Literature 2, a long axis and a short axis that is shorter than the long axis and is perpendicular to the long axis are defined at an end portion of the valve-seat opening. A fuel chamber having a long axis longer than the inner diameter of the valve-seat opening and a short axis shorter than the inner diameter of the valve-seat opening is provided. Inside the fuel chamber, the injection holes are formed.
As a result, the fuel flows to the radially outer side with respect to the valve-seat axial center to form a swirl flow in the combustion chamber and form the injected fuel into the thinner film. In this manner, the promotion of the atomization is realized.
By the way, in order to clarify a mechanism of the atomization of the fuel spray, the fuel injected from the injection holes is first photographed in an enlarged manner. As a result, it is proven that the fuel breakup process proceeds from “liquid film” through “liquid thread” to “droplet” by a force for diffusing the fuel that overcomes a surface tension.
Moreover, it is also proven that the effects of the surface tension become greater after the fuel once becomes the liquid droplets and therefore the breakup less liable to occur from then on.
Thus, it is proven that a higher degree of atomization can be achieved when the “thin liquid film” with smaller fuel turbulence is injected from the injection holes and the liquid film is further spread thinly to be then broken up. On the contrary, when the turbulence occurs in the fuel flow, the breakup occurs in a state of a “thick liquid film” before the fuel liquid film is spread thinly. Therefore, the droplets after the breakup also become larger.
Moreover, in the fuel injection valve including the plurality of injection holes formed through the injection-hole plate, which are arranged on the radially outer side of the valve-seat opening inner wall, and the cavity that brings the valve-seat opening inner wall and the injection holes into communication with each other, when the valve is opened, the fuel spreads radially from the valve-seat axial center inside the cavity to flow toward the center of the injection hole.
Therefore, when the spray aggregate is sprayed from the fuel injection valve in a plurality of directions, an orientation of the flow of the fuel flowing into the injection hole suddenly changes in the injection hole toward a desired injection direction. Therefore, the swirl flow is formed in each of the injection holes. At this time, the swirl flow generated in the injection holes is to spread the fuel into the thin liquid film without causing the turbulence in the fuel flow. Therefore, it is proven that the injected particle diameter can be reduced.
In neither Patent Literature 1 nor Patent Literature 2, however, the clarified atomization mechanism described above is appropriately used.